It is known to produce fine fibers by vapor-growing, for example, vapor-grown carbon fibers. A substrate such as alumina and the like or graphite or the like is placed in an electric furnace, in which is provided an ultra-fine particle catalyst of iron, nickel, or cobalt as a core for growth of carbon fibers, over which is introduced a mixed gas of a hydro-carbon gas such as benzene or the like and a hydrogen carrier gas so as to decompose the hydrocarbon at a temperature of 800.degree. to 1300.degree. C., whereby carbon fibers can be grown on the substrate.
However, such a production means of vapor-growth in which the substrate is used has had many problems such that the yield is poor because temperature conditions and reaction conditions at the substrate surface are not uniform, and the productivity is also poor because it is troublesome to withdraw fibers continuously.
From such a viewpoint, the present applicant has already used an organic metal compound which is evaporated to make a matal catalyst in a gaseous phase to give a novel procedure, whereby it has been made possible to omit three operations of dispersing a conventional catalyst on a substrate, reducing in a furnace, and withdrawing formed fibers toward the outside of the furnace, and a gas of an organic compound as a carbon source in the gaseous phase and a gas of an organic transition metal compound as a catalyst source are subjected to thermal decomposition in an electric furnace heated to a required temperature, whereby the catalyst and the carbon fiber are continuously produced in a floating state to develop a means which has been filed as a patent application, resulting in allowance as a patent (Japanese Patent Publication No. 62-49363).
However, also in the means for producing vapor-grown fibers in relation to the above mentioned patent, accumulation of fibers in the furnace occurred, and it was difficult to make sufficiently continuous fibers.
As a reason therefor, it is postulated that catalyst particles generated in the electric furnace, for example, iron fine particles generated by decomposition of a gas of ferrocene (a state of which is not clear as pure iron, carbide of iron, or any one of solid, liguid, or gas) adhere to a wall of the furnace, which form fibers in a manner of the substrate method, and the fibers trap floating fibers to increase the accumulation. And as another one, it is postulated that the floating fibers adhere to the wall of the furnace for some reason, and the floating fibers deposit due to gravity especially in the case of a furnace of the horizontal type.
When fibers are accumulated in the furnace, there is a problem that granular carbon is formed probably because a gas stream in the furnace becomes irregular, and there is caused irregularity of products because fibers accumulated at the early stage of operation continue the growth in the diameter direction during the operation, which provides a great difference in diameter between fibers which have gone out from the furnace in a short period and fibers which have accumulated at the late stage of operation. Therefore, it was necessary that the accumulated fibers were removed at a suitable cycle.
A gas containing hydrogen is generally used as the carrier gas, so that the fibers must be scraped after replacing the inside of the furnace with a noncombustible gas such as nitrogen or the like, and hence no satisfactory operation performance is obtained. As a countermeasure therefor, a method has also been proposed in which a scraping unit of the screw type is provided at the inside of the furnace, and the accumulated fibers are continuously scraped during operation, however, there is a problem that the catalyst fine particles adhere to the screw to allow fibers to grow, and the operation performance inversely decreases due to elimination thereof. In addition, when a flow rate of gas is made large, or when the furnace is made short to make a staying period at the heating portion short, then a ratio of accumulation of fibers in the furnace decreases, but the thickness growth of fibers is insufficient, resulting in that fibers eluted from the furnace have extremely thin diameters.
Gary G. Tibbets et al. described in the specification of U.S. Pat. No. 4,565,684 that a concentration of a raw material gas is made low during the forming stage of fibers so as to make the number of fibers formed and their length as great as possible, and then the concentration of the raw material is made high so as to thicknen the fibers, whereby the production efficiency of fiber is increased. In this case, the above mentioned U.S. Patent considers the substrate method only, however, the substrate method basically has a poor productivity as described above. The present invention is directed to a problem of clogging due to fibers in a furnace in the fluidization gaseous phase method which basically has a high productivity, wherein no way of solution is given even by analogy to the method of the above mentioned U.S. patent.
The present inventors have done various investigations on the improvement of the operation performance and the improvement of the degree of freedom of decision on fiber diameter in the production of fine fibers by the vapor-growing method, and found that although the formation of fibers and the thickness growth thereof were conventionally performed at the same time in one furnace, the problems can be solved by separating the both steps, resulting in accomplishment of the present invention.